But up to 39% of people who develop COPD have never smoked, and their lung cancer risk has been unclear due to a lack of studies.

To learn more, researchers analyzed data from more than 338,500 adults, ages 40 to 84, in South Korea who had no history of lung cancer. Their health was tracked for an average of seven years.

During that time, 1,834 participants developed lung cancer -- 290 of them had COPD.

Compared with people who never smoked and didn't have COPD, current and former smokers without COPD were about twice as likely to develop lung cancer. Current and former smokers with COPD had a six times higher risk.

Among people who'd never smoked, those with COPD had a more than 2.5 times the risk of lung cancer than those without COPD.

Lung cancer risk in nonsmokers with COPD was similar to that of smokers without COPD, according to the study published online April 2 in the journal Thorax.

"Given that poor lung function in COPD is often a barrier to optimal lung cancer treatment due to increased risk of treatment related morbidities, our study suggests that early detection of lung cancer in COPD patients may reduce the risk of treatment complications," the authors wrote.

Juhee Cho of the Center for Clinical Epidemiology at Samsung Medical Center in Seoul, South Korea led the study.

Her team said the findings suggest that COPD is a strong independent risk factor for lung cancer.